The invention relates to a heating device for filter elements of a particle filter according to the preamble of claim 1 and to a particle filter according to the preamble of claim 20.
Heating devices are known, in which the filter is a carbon black or soot filter and comprises a stack of filter disks. Several circumferentially distributed tubular heaters of a conventional nature pass in externally engaging manner axially with respect to the disk stack. They heat the filter disk or the soot deposited thereon up to the ignition temperature thereof, so that the soot can burn and the filter is recleaned. This arrangement suffers from the major disadvantage that the necessary heating capacity is very high for obtaining a satisfactory burning off of the soot.
The problem of the invention is to provide a heating device of the aforementioned type and a particle filter provided therewith enabling the burning of the deposited particles, e.g. soot, in an advantageous, efficient manner and as completely as possible.
This problem is solved by a heating device having the features of claim 1 and a particle filter having the features of claim 20. Further advantageous and preferred developments of the invention appear in the further claims and are described in greater detail hereinafter. By express reference the wording of the claims forms part of the content of the description. In this application the word comprise does not have the meaning of is limited to, but including.
According to the invention a heating device has a tubular heater, which has an outer tube and at least one heating conductor located therein. Such a tubular heater can fundamentally be constructed in similar manner to known tubular heaters, which are used in many fields of technology, e.g. in domestic ovens and the like. A preferred field of application for a heating device according to the invention is soot filters with disk-like, stacked filter elements. The invention can also be used for other filter types, e.g. advantageously also for ceramic filters. These ceramic filters can have a monolithic ceramic honeycomb body. According to the invention the tubular heater has a small diameter, particularly compared with conventional tubular heaters and as a result it can pass in the vicinity of the filter area or the surface of a filter element.
In a previously described filter disk stack or the like, it is possible as a result of the small diameter to place the tubular heater between the filter disks or very close to the filter surface. The path of the tubular heater is chosen in such a way that through a corresponding construction or bending the tubular heater covers a surface area. The surface-covering path of the tubular heater can e.g. be in meander-like form. Further possibilities are provided by curved paths. Through the combination of the limited distance from the filter area and the flat path, a burning off of deposited soot or other particles from filter disks or elements is very advantageously possible and can be achieved with a limited energy expenditure.
A small diameter is understood to mean a diameter of less than 3 mm. Conventional tubular heaters have a diameter of 6.5 mm or more. With particular advantage the tubular heater is thinner than 2 mm. According to a preferred embodiment the diameter is only approximately 1.5 mm. Thus, the spacing between two filter elements between which such a tubular heater is placed, is a few millimetres, e.g. 2 to 4 mm. This permits a confined arrangement or stacking of several filter elements with interposed heating means.
A tubular heater can be constructed in such a way that it only covers part of an area or surface of a soot filter element. This fraction of the surface area is preferably a quarter or a half. A specific design possibility is a tubular heater or a heating device in the form of a semicircle or quadrant. This means that the tubular heater is constructed in such a way that it covers or heats such a surface area without precisely corresponding to the outer contours thereof. This is particularly advantageous with round or circular filter elements.
The material for the heating conductor can be a chromium-containing material, e.g. a chromium, nickel or similar, conventionally used alloy. Particular preference is given to FeCrAl. The outer tube is also advantageously made from the latter.
The heating conductor diameter can be advantageously adapted to the dimensions of the tubular heater and the electrical requirements. An advantageous diameter is between 0.2 and 1 mm, more particularly approximately 0.5 mm. The heating conductor is preferably substantially straight or uncoiled. It preferably passes linearly in the tubular heater.
It is particularly advantageous if the tubular heater has a limited or low mass. This can be achieved on the one hand by a thin outer tube and also with respect to the wall thickness and the overall diameter. A further possibility is a correspondingly low-mass insulating material between the heating conductor and the outer tube. It is particularly advantageous to give the tubular heater a low mass, because a lower mass gives rise to a faster glowing or heating time. For this purpose it is possible to use for the heating device or tubular heater materials aiding a low thermal inertia, i.e. a low thermal capacity. In addition, this leads to a weight saving. As a result of a small diameter the volume is small, so that there is a reduced encroachment on the filter volume or filter capacity.
The further advantage of a low-mass tubular heater is that on engaging or striking against the surface of a filter element it gives rise to no serious damage, because the kinetic energy of such a blow is smaller with a lower mass. A light tubular body is also less susceptible to vibrations, such as can arise when using soot filters in motor vehicles or the like.
An insulation between the heating conductor and the outer tube can have a very fine-grain powder. Advantageously this is of a finer grain nature than in conventional tubular heaters. It can also be compacted in order to ensure a reliable insulation, particularly with curved tubular heaters. An advantageous material is magnesium oxide.
In order to electrically connect the tubular heater or heating conductor, it can be connected or welded to a connecting lug. This advantageously takes place in the area where the heating conductor passes out of the outer tube. Such a connecting lug can have a widely varying design in order to create a simple and reliable connection possibility. For improving the connection there is an additional tubular sleeve or a short tube over the connection of the heating conductor to the connecting lug. Thus, the connection is both fixed and protected against external mechanical influences. Within this tubular sleeve or an envelope of the connection can be introduced an embedding pourable compound for insulation purposes. Ceramic materials are suitable and they either dry out in a conventional manner or are not burned. Another possibility is glass or a glass duct. Another advantage of such an additional sleeve or the like is that over the larger mass and surface a cooling of the heating device or tubular heater can take place in this area. This is e.g. advantageous in those cases where the connection is not located in the cooling gas flow within a filter.
The outer tube can be used as an electric return conductor. For this purpose e.g. the heating conductor can be welded at one end to the outer tube, which reduces the connection costs and fault susceptibility.
A method for the manufacture of such a tubular heater can be essentially based on known methods for the production of known tubular heaters.
A particle filter according to the invention has several filter elements stacked in a union. The filter elements have filter surfaces where, on passing through the same, particles or soot are separated from a gas. With at least one filter area is associated a heating device, in the manner described hereinbefore and is used for burning off particles deposited on the filter surface and consequently cleans the filter element or face. According to the invention the heating device in the form of a tubular heater does not pass outside on the edge of a filter surface, but instead partly covers the same or runs very close thereto. This makes it possible to produce the energy supply much more directly to the filter surface and consequently e.g. to improve soot burning. This is made possible by the above-described, very thin tubular heaters according to the invention. In conventional tubular heaters for fitting between the filter elements the spacing thereof must be sufficiently large that the overall size of the filter for a given filter performance would be excessive. For further reducing the diameter or size the tubular heater can be pressed flat and e.g. have an oval cross-section.
A tubular heater can be constructed in such a way that it does not cover the entire surface of a filter element. In the case of a predetermined supply voltage of 12 V, e.g. in a motor vehicle, this could lead to high current intensities. Therefore a tubular heater advantageously only covers part of a filter element, e.g. a quarter or preferably half.
The filter elements in the above-described particle filter are advantageously disk-like and can in particular be constructed as double disks with an inner space. These filter elements are stacked on one another and a gap is advantageously provided between the filter surfaces. In said gap or between in each case two facing filter faces is located a heating device with a tubular heater according to the invention. This tubular heater heats both filter surfaces, preferably in a uniform manner.
A tubular heater can be used as a spacer for a filter. This is appropriate e.g. in the case of a filter formed from several filter elements, also for improving the stability thereof. Such a use is particularly preferred with a stack filter having several filter elements. With a corresponding construction, the tubular heater can also be used for fixing filter elements.
These and further features can be gathered from the claims, the description and the drawings and the individual features, either individually or in the form of subcombinations, can be used in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions for which protection is claimed here. The subdivision of the application into individual sections and the subheadings in no way restrict the general validity of the statements made thereunder.